This invention generally relates to the problem of energy induced onto abandoned implanted leads during medical diagnostic procedures such as magnetic resonant imaging (MRI). Specifically, the radio frequency (RF) pulsed field of MRI can couple to an implanted lead in such a way that electromagnetic forces (EMFs) are induced in the lead. The amount of energy that is induced is related to a number of complex factors, but in general, is dependent upon the local electric field that is tangent to lead and the integral of the electric field strength along the lead. In certain situations, these EMFs can cause currents to flow into distal electrodes or in the electrode interface with body tissue. It has been documented that when this current becomes excessive, that overheating of said lead or its associated electrode or overheating of the associated interface with body tissue can occur. There have been cases of damage to such body tissue which has resulted in loss of capture of cardiac pacemaking pulses, tissue damage, severe enough to result in brain damage or multiple amputations, and the like. The present invention relates generally to methods of redirecting said energy to a novel energy dissipating abandoned lead proximal end cap rather than the lead body or a distal tip electrode-to-tissue interface.
There are many reasons why cardiac rhythm device lead wires are abandoned. These include loss of pacing capture or a high impedance at the distal electrode to tissue interface. Another reason includes lead breakage or damage to lead insulation. Yet another reason would be simply due to replacement and relocation of the active implantable medical device (AIMD). Removal of implanted lead wires is not an easy process, particularly after they've been implanted for a long period of time. Reference is made to a paper given at the 28th Annual Scientific Sessions of the Heart Rhythm Society, in Session 113 on Friday, May 11, 2007 by Dr. Bruce L. Wilkoff, M. D. of the Cleveland Clinic Foundation and was entitled, ICD LEAD EXTRACTION OF INFECTED AND/OR REDUNDANT LEADS. The slides from that paper are incorporated herein by reference and will be referred to again simply as the Wilkoff reference. Referring to various Figures in the paper, one can see the amount of tissue that is adhering to the leads as they are extracted. During extraction procedures, there are various cutting tools and laser tools that are slipped down over the lead that are used to dislodge the lead from surrounding tissue growth. This is a very delicate process because it's a tortuous path. For example, if the laser or mechanical cutting tool were to penetrate while going around a corner of an artery wall, this would result in a life-threatening situation for the patient. Accordingly, lead wires are often simply abandoned and then clipped off or capped and left inside the patient. As mentioned, the above examples were for implanted cardiac rhythm device leads. There are also many reasons why leads are abandoned for neurostimulators and other types of AIMDs. Spinal cord, deep brain or cochlear leads are often abandoned simply because the electrodes are so difficult to extract. The present invention is applicable to all types of abandoned implanted leads.
It's also been demonstrated in the literature that abandoned lead wires can be quite dangerous during magnetic resonance imaging (MRI) procedures. That is, the energy that is coupled from the pulsed RF field of the magnetic resonance imaging field creates significant energy in the lead wire system. In most pacemakers and cardioverter defibrillators, there is an EMI filter that's present at the point of lead wire ingress through the hermetic titanium housing of the device. These prior art feedthrough capacitors (or monolithic chip capacitors) form a fairly low impedance at MRI RF pulse frequencies. The RF pulsed frequency for a 1.5 Tesla MR scanner is approximately 64 MHz. For a 3 Telsa scanner, the RF pulsed frequency is approximately 128 MHz. The capacitive reactance of the prior art EMI feedthrough capacitors is generally below 2 ohms at these frequencies. Therefore, when the pacemaker or ICD is plugged into the proximal end of the lead wire, much of the RF energy from MRI is shunted to the titanium can or housing of the AIMD. This is why there have been some reports of warming of the pectoral pocket during MR scans. One is referred to a paper given at Heart Rhythm 2007 by Dr. Rod Gimbal. He reported on a number of ICD patients, including one patient who reported warming of the pectoral pocket area during the MR scan. When the physician placed his hand over the patient's AIMD in the pocket area, the doctor himself could feel the heat radiating into his own hand. This could have been caused by MR gradient field eddy current heating, however, it is more likely that the heating was caused by transfer of energy through the feedthrough capacitor to the housing of the cardiac pacemaker.
Accordingly, when a pacemaker or ICD is unplugged and the abandoned lead(s) is capped or cut off, there is no path for the energy to escape at the proximal end into the surrounding tissues. Instead, what happens is the energy instead dissipates at the still-connected distal tip electrode or distal ring electrode. Severe overheating has been documented in the literature, including burns to cardiac tissue. Therefore, what is needed is an abandoned lead cap that is capable of transferring energy at the proximal end. For example, for a cardiac pacemaker, this would typically be in the pectoral pocket where the pacemaker was previously removed. In some cases, a new pacemaker with new leads is implanted in the same pectoral pocket as the abandoned leads. The pectoral pocket, which embodies fat and muscle tissues, is not nearly as sensitive to thermal injury as compared to myocardial tissue, the spinal cord, or deep brain tissue. For all of these types of AMIDs, pectoral implants are common. Similar analogies can be made for spinal cord stimulators and other types of neuromodulation systems. In other words, when one makes the choice, it will be better to slightly overheat the pectoral muscle than it would to overheat the distal electrode tissue interface. It will be obvious that overheating of myocardial, nerves or brain tissue can be debilitating or even life threatening. It is also a feature of the present invention to dissipate said energy over a large enough surface area of a novel abandoned lead cap such as to prevent overheating to the point where temperature rise would result in thermal injury.
MRI is one of medicine's most valuable diagnostic tools. MRI is, of course, extensively used for imaging, but is also used for interventional medicine (surgery). In addition, MRI is used in real time to guide ablation catheters, neurostimulator tips, deep brain probes and the like. An absolute contraindication for pacemaker or neurostimulator patients means that these patients are excluded from MRI. This is particularly true of scans of the thorax and abdominal areas. Because of MRI's incredible value as a diagnostic tool for imaging organs and other body tissues, many physicians simply take the risk and go ahead and perform MRI on a pacemaker patient. The literature indicates a number of precautions that physicians should take in this case, including limiting the power of the MRI RF Pulsed field (Specific Absorption Rate—SAR level), programming the pacemaker to fixed or asynchronous pacing mode, and then careful reprogramming and evaluation of the pacemaker and patient after the procedure is complete. There have been reports of latent problems with cardiac pacemakers or other AIMDs after an MRI procedure sometimes occurring many days later. Moreover, there are a number of recent papers that indicate that the SAR level is not entirely predictive of the heating that would be found in implanted leads or devices. For example, for magnetic resonance imaging devices operating at the same magnetic field strength and also at the same SAR level, considerable variations have been found relative to heating of implanted leads. It is speculated that SAR level alone is not a good predictor of whether or not an implanted device or its associated lead system will overheat.
There are three types of electromagnetic fields used in an MRI unit. The first type is the main static magnetic field designated B0 which is used to align protons in body tissue. The field strength varies from 0.5 to 3.0 Tesla in most of the currently available MRI units in clinical use. Some of the newer MRI system fields can go as high as 4 to 5 Tesla. At the recent International Society for Magnetic Resonance in Medicine (ISMRM), which was held on 5-6 Nov. 2005, it was reported that certain research systems are going up as high as 11.7 Tesla and will be ready sometime in 2010. This is over 100,000 times the magnetic field strength of the earth. A static magnetic field can induce powerful mechanical forces and torque on any magnetic materials implanted within the patient. This would include certain components within the cardiac pacemaker itself and/or lead systems. It is not likely (other than sudden system shut down) that the static MRI magnetic field can induce currents into the pacemaker lead system and hence into the pacemaker itself. It is a basic principle of physics that a magnetic field must either be time-varying as it cuts across the conductor, or the conductor itself must move within a specifically varying magnetic field for currents to be induced.
The second type of field produced by magnetic resonance imaging is the pulsed RF field which is generated by the body coil or head coil. This is used to change the energy state of the protons and elicit MRI signals from tissue. The RF field is homogeneous in the central region and has two main components: (1) the electric field is circularly polarized in the actual plane; and (2) the H field, sometimes generally referred to as the net magnetic field in matter, is related to the electric field by Maxwell's equations and is relatively uniform. In general, the RF field is switched on and off during measurements and usually has a frequency of 21 MHz to 64 MHz to 128 MHz depending upon the static magnetic field strength. The frequency of the RF pulse for hydrogen scans varies by the Lamor equation with the field strength of the main static field where: RF PULSED FREQUENCY in MHz=(42.56) (STATIC FIELD STRENGTH IN TESLA). There are also phosphorous and other types of scanners wherein the Lamor equation would be different. The present invention applies to all such scanners.
The third type of electromagnetic field is the time-varying magnetic gradient fields designated BX, BY, BZ, which are used for spatial localization. These change their strength along different orientations and operating frequencies on the order of 1 kHz. The vectors of the magnetic field gradients in the X, Y and Z directions are produced by three sets of orthogonally positioned coils and are switched on only during the measurements. In some cases, the gradient field has been shown to elevate natural heart rhythms (heart beat). This is not completely understood, but it is a repeatable phenomenon. The gradient field is not considered by many researchers to create any other adverse effects.
It is instructive to note how voltages and electro-magnetic interference (EMI) are induced into an implanted lead system. At very low frequency (VLF), voltages are induced at the input to the cardiac pacemaker as currents circulate throughout the patient's body and create voltage drops. Because of the vector displacement between the pacemaker housing and, for example, the tip electrode, voltage drop across the resistance of body tissues may be sensed due to Ohms Law and the circulating current of the RF signal. At higher frequencies, the implanted lead systems actually act as antennas where voltages (EMFs) are induced along their length. These antennas are not very efficient due to the damping effects of body tissue; however, this can often be offset by extremely high power fields (such as MRI pulsed fields) and/or body resonances.
Magnetic field coupling into an implanted lead system is based on loop areas. For example, in an AIMD abandoned lead, there is a loop formed by the lead as it comes from the abandoned lead proximal tip to its distal tip electrode, for example, located in the right ventricle. The return path is through body fluid and tissue generally straight from the tip electrode in the right ventricle back up to the abandoned lead cap or end. This forms an enclosed area which can be measured from patient X-rays in square centimeters. Per ANSI/AAMI National Standard PC69, the average loop area is 200 to 225 square centimeters. This is an average and is subject to great statistical variation. For example, in a large adult patient with an abdominal pacemaker implant, the implanted loop area is much larger (around 400 square centimeters).
Relating now to the specific case of MRI, the magnetic gradient fields would be induced through enclosed loop areas. However, the pulsed RF fields, which are generated by the body coil, would be primarily induced into the lead system by antenna action. Subjected to RF frequencies, the lead itself can exhibit complex transmission line behavior.
At the frequencies of interest in MRI, RF energy can be absorbed and converted to heat. The power deposited by RF pulses during MRI is complex and is dependent upon the power (Specific Absorption Rate (SAR) Level) and duration of the RF pulse, the transmitted frequency, the number of RF pulses applied per unit time, and the type of configuration of the RF transmitter coil used. The amount of heating also depends upon the volume of tissue imaged, the electrical resistivity of tissue and the configuration of the anatomical region imaged. There are also a number of other variables that depend on the placement in the human body of the AIMD and the length and trajectory of its associated lead(s). For example, it will make a difference how much EMF is induced into a pacemaker lead system as to whether it is a left or right pectoral implant. In addition, the routing of the lead and the lead length are also very critical as to the amount of induced current and heating that would occur. Also, distal tip design is very important as it can heat up due to MRI RF induced energy. The cause of heating in an MRI environment is twofold: (a) RF field coupling to the lead can occur which induces significant local heating; and (b) currents induced between the distal tip and tissue during MRI RF pulse transmission sequences can cause local Ohms Law heating in tissue next to the distal tip electrode of the implanted lead. The RF field of an MRI scanner can produce enough energy to induce RF voltages in an implanted lead and resulting currents sufficient to damage some of the adjacent myocardial tissue. Tissue ablation (destruction resulting in scars) has also been observed. The effects of this heating are not readily detectable by monitoring during the MRI. Indications that heating has occurred would include an increase in pacing threshold, venous ablation, Larynx or esophageal ablation, myocardial perforation and lead penetration, or even arrhythmias caused by scar tissue. Such long term heating effects of MRI have not been well studied yet for all types of AIMD lead geometries. There can also be localized heating problems associated with various types of electrodes in addition to tip electrodes. This includes ring electrodes or pad electrodes. Ring electrodes are commonly used with a wide variety of abandoned implanted device leads including cardiac pacemakers, and neurostimulators, and the like. Pad electrodes are very common in neurostimulator applications. For example, spinal cord stimulators or deep brain stimulators can include a plurality of pad electrodes to make contact with nerve tissue. A good example of this also occurs in a cochlear implant. In a typical cochlear implant there would be sixteen pad electrodes placed up into the cochlea. Several of these pad electrodes make contact with auditory nerves.
Just variations in the pacemaker lead length and implant trajectory can significantly affect how much heat is generated. A paper entitled, HEATING AROUND INTRAVASCULAR GUIDEWIRES BY RESONATING RF WAVES by Konings, et al., Journal of Magnetic Resonance Imaging, Issue 12:79-85 (2000), does an excellent job of explaining how the RF fields from MRI scanners can couple into implanted leads. The paper includes both a theoretical approach and actual temperature measurements. In a worst-case, they measured temperature rises of up to 74 degrees C. after 30 seconds of scanning exposure. The contents of this paper are incorporated herein by reference.
The effect of an MRI system on the abandoned leads of pacemakers, ICDs, neurostimulators and the like, depends on various factors, including the strength of the static magnetic field, the pulse sequence, the strength of RF field, the anatomic region being imaged, and many other factors. Further complicating this is the fact that each patient's condition and physiology is different and each lead implant has a different length and/or implant trajectory in body tissues. Most experts still conclude that MRI for the pacemaker patient should not be considered safe.
It is well known that many of the undesirable effects in an abandoned implanted lead system from MRI and other medical diagnostic procedures are related to undesirable induced EMFs in the lead system and/or RF currents in its distal tip (or ring) electrodes. This can lead to overheating of body tissue at or adjacent to the distal tip.
Distal tip electrodes can be unipolar, bipolar and the like. It is very important that excessive current not flow at the interface between the lead distal tip electrode and body tissue. In a typical cardiac pacemaker, for example, the distal tip electrode can be passive or of a screw-in helix type as will be more fully described. In any event, it is very important that excessive RF current not flow at this junction between the distal tip electrode and for example, myocardial or nerve tissue. Excessive current at the distal electrode to tissue interface can cause excessive heating to the point where tissue ablation or even perforation can occur. This can be life threatening for cardiac patients. For neurostimulator patients, such as deep brain stimulator patients, thermal injury can cause permanent disability or even be life threatening. Similar issues exist for spinal cord stimulator patients, cochlear implant patients and the like.
A very important and life-threatening problem is to be able to control overheating of abandoned implanted leads during an MRI procedure. A novel and very effective approach to this is to first install parallel resonant inductor and capacitor bandstop filters at or near the distal electrode of implanted leads. For cardiac pacemaker, these are typically known as the tip and ring electrodes. One is referred to U.S. Pat. No. 7,363,090; US 2007/0112398 A1; US 2008/0071313 A1; US 2008/0049376 A1; US 2008/0024912 A1; US 2008/0132987 A1; and US 2008/0116997 A1, the contents of all of which are incorporated herein. Referring now to US 2007/0112398 A1, the invention therein relates generally to L-C bandstop filter assemblies, particularly of the type used in active implantable medical devices (AIMDs) such as cardiac pacemakers, cardioverter defibrillators, neurostimulators and the like, which raise the impedance of internal electronic or related wiring components of the medical device at selected frequencies in order to reduce or eliminate currents induced from undesirable electromagnetic interference (EMI) signals.
U.S. Pat. No. 7,363,090 and US 2007/0112398 A1 show resonant L-C bandstop filters placed at the distal tip and/or at various locations along the medical device leads or circuits. These L-C bandstop filters inhibit or prevent current from circulating at selected frequencies of the medical therapeutic device. For example, for an MRI system operating at 1.5 Tesla, the pulse RF frequency is 64 MHz, as described by the Lamour Equation for hydrogen. The L-C bandstop filter can be designed to resonate at or near 64 MHz and thus create a high impedance (ideally an open circuit) in the lead system at that selected frequency. For example, the L-C bandstop filter, when placed at the distal tip electrode of a pacemaker lead, will significantly reduce RF currents from flowing through the distal tip electrode and into body tissue. The L-C bandstop filter also reduces EMI from flowing in the leads of a pacemaker, for example, thereby providing added EMI protection to sensitive electronic circuits. In general, the problem associated with abandoned leads is minimized when there is a bandstop filter placed at or adjacent to its distal tip electrodes. However, experiments have shown that even when such a bandstop filter is present, if the AIMD is disconnected, distal tip heating can still occur. This is generally due to the fact that prior art EMI filters located in the AIMD shunt some of the MRI induced RF energy out of the leads to the generally conductive housing of the AIMD. In this case, the AIMD housing, such as the housing of a cardiac pacemaker, acts as an important energy dissipating surface (EDS surface). In general, when the housing of the AIMD acts as an EDS surface, it does not rise in temperature very much due to its very large surface area and energy dissipating surface. However, when the lead is abandoned, in other words, the AIMD is removed; the proximal end of the lead is now terminated either in body tissue or in an insulated abandoned lead cap. It no longer is associated with an EDS surface or even a means to couple or divert energy to an EDS surface. Accordingly, MRI induced RF energy reflects off this open circuit and goes right on back to the distal electrodes where it can bounce back and forth and cause overheating, even when a distal bandstop filter is present. It will be appreciated that all of the embodiments described therein are equally applicable to a wide range of other implantable and external medical devices, including deep brain stimulators, spinal cord stimulators, drug pumps, probes, catheters and the like.
Electrically engineering a capacitor in parallel with an inductor is known as a bandstop filter or tank circuit. It is also well known that when a near-ideal L-C bandstop filter is at its resonant frequency, it will present a very high impedance. Since MRI equipment produces very large RF pulsed fields operating at discrete frequencies, this is an ideal situation for a specific resonant bandstop filter. Bandstop filters are more efficient for eliminating one single frequency than broadband filters. Because the L-C bandstop filter is targeted at this one frequency, it can be much smaller and volumetrically efficient.
A major challenge for designing an L-C bandstop filter for human implant is that it must be very small in size, biocompatible, and highly reliable. Coaxial geometry is preferred. The reason that coaxial is preferred is that implanted leads are placed at locations in the human body primarily by one of two main methods. These include guide wire lead insertion. For example, in a cardiac pacemaker application, a pectoral pocket is created. Then, the physician makes a small incision between the ribs and accesses the subclavian vein. The pacemaker leads are stylus guided/routed down through this venous system through the superior vena cava, through the right atrium, through the tricuspid valve and into, for example, the right ventricle. Another primary method of implanting leads (particularly for neurostimulators) in the human body is by tunneling. In tunneling, a surgeon uses special tools to tunnel under the skin and through the muscle, for example, up through the neck to access the Vagus nerve or the deep brain. In both techniques, it is very important that the leads and their associated electrodes at the distal tips be very small. US2007/0112398 A1 solves these issues by using very novel miniature coaxial or rectilinear capacitors that have been adapted with an inductance element to provide a parallel L-C bandstop filter circuit.
The value of the capacitance and the associated parallel inductor can be adjusted to achieve a specific resonant frequency (SRF). The bandstop filters described in US 2007/0112398 A1 can be adapted to a number of locations within the overall implantable medical device system. That is, the L-C bandstop filter can be incorporated at or near any part of the medical device implanted lead system or at or adjacent to the distal tip electrodes. In addition, the L-C bandstop filter can be placed anywhere along the implanted lead system.
The L-C bandstop filters are also designed to work in concert with an EMI filter which is typically used at the point of lead ingress and egress of the active implantable medical device. For example, see U.S. Pat. No. 5,333,095; U.S. Pat. No. 5,905,627; U.S. Pat. No. 5,896,627; and U.S. Pat. No. 6,765,779, the contents of all being incorporated herein by reference. All four of these documents describe low pass EMI filter circuits. Accordingly, the L-C bandstop filters, as described in U.S. Pat. No. 7,393,090, entitled BANDSTOP FILTER EMPLOYING A CAPACITOR AND INDUCTOR TANK CIRCUIT TO ENHANCE MRI COMPATIBILITY OF ACTIVE IMPLANTABLE MEDICAL DEVICES, are designed to be used in concert with these prior art low pass filters. However, when an AIMD lead is abandoned, the filter capacitors, as previously described, are no longer connected to the lead. Bandstop filters, in accordance with U.S. Patent Application Publication No. US2007/0112398 A1, work particularly well when the proximal lead is connected to a pacemaker or ICD that has a feedthrough capacitor EMI filter. When the AIMD is disconnected, there is no place for the energy to go at the proximal end. Accordingly, the energy is reflected back to the distal tip. Recent testing by the inventors demonstrates that in certain lead configurations, even with a bandstop filter present, excessive heating at the distal tip electrode can still occur.
When one performs MRI testing on an abandoned lead system, one first establishes a controlled measurement. That is, with worst-case MRI equipment settings and a worst-case location within the MRI bore, and a worst-case lead configuration, one can measure heating using fiber optic probes at the distal electrodes. Temperature rises of 30 to over 60 degrees C. have been documented. When one takes the same control lead and places miniature bandstop filters in accordance with U.S. Pat. No. 7,363,090 or US 2007/0112398 A1, one finds that substantially less MRI induced energy is directed to distal electrodes greatly reducing their tendency to overheat. In fact, in many measurements made by the inventors, temperature rises of over 30 degrees C. have been reduced to less than 3 degrees C. However, a secondary problem has been discovered. That is, the implanted lead acts very much as like a transmission line. When one creates a very high impedance at the distal electrode to tissue interface by installation of a resonant bandstop filter as described in U.S. Pat. No. 7,038,900 and as further described in US 2007/0112398 A1, there is created an almost open circuit which is the equivalent of an unterminated transmission line. This causes a reflection of MRI induced RF energy back towards the proximal end where the AIMD (for example, a pacemaker) would have been connected. However, for an abandoned lead, this creates an open circuit at the proximal end with no place for the energy to escape. Therefore, this energy can be reflected back and forth resulting in temperature rises along the lead and more particularly at the distal electrode to tissue interface. In order to completely control the induced energy in an abandoned implanted lead system, one must take a system approach. In particular, a methodology is needed whereby energy can be dissipated from the lead system at the proximal end in a way that does not cause overheating either at the distal electrode interface or at the proximal end cap. Maximizing energy transfer from an implanted lead is more thoroughly described in U.S. patent Ser. No. 12/686,137, the contents of which are incorporated herein by reference.
Accordingly, there is a need for controlling the induced energy in an implanted abandoned lead system. Moreover, there is a need for novel tuned RF diverting circuits coupled to one or more energy or heat dissipation surfaces associated with an abandoned lead cap, which are preferably frequency selective and are constructed of passive components. Such circuits are needed to prevent MRI induced energy from reaching the distal tip electrode or its interface with body tissue. By redirecting said energy to an energy dissipation surface distant from the distal electrodes, this minimizes or eliminates hazards associated with overheating of said lead and/or its distal electrodes during diagnostic procedures, such as MRI. For maximum RF energy transfer out of the lead, frequency selective diverter circuits are needed which decouple and transfer energy which is induced onto implanted leads from the MRI pulsed RF field to an energy dissipating surface associated with an abandoned lead cap. The present invention fulfills these needs and provides other related advantages.